Bimodal gear lubricant formulation

ABSTRACT

The present invention provides a hypoid axle bimodal gear lubricant formulation comprising: (a) a base oil of lubricating viscosity, (b) at least one viscosity index improver, (c) a friction modifier, and (d) an antiwear additive; wherein said bimodal gear lubricant formulation produces a gel permeation chromatogram having at least a first peak representative of the base oil and at least a second peak representative of the viscosity index improver, and wherein said base oil has a viscosity at 100° C. in the range of from about 2 centistokes to about 8 centistokes, and wherein said undilute viscosity index improver has a viscosity in the range of from about 600 centistokes to about 45,00 centistokes.

FIELD OF THE INVENTION

[0001] The present invention relates to bimodal gear lubricant formulations having improved industrial and automotive fuel economy properties.

BACKGROUND OF THE INVENTION

[0002] The primary function of a gear lubricant is to provide a high degree of reliability and durability in the service life of gear equipment. Gear lubricants may also contribute to improving the fuel economy of vehicles by improving the axle efficiency. See, for example, O'Connor et al., The Relationship Between Laboratory Axle Efficiency and Vehicle Fuel Consumption (SAE Paper No. 811206).

[0003] A key performance need for modem axle fluids, especially those used in truck and sport utility vehicle applications, is the ability to handle high stress load under towing conditions. Conventional axle fluids, including both mineral oil and synthetic oil, cannot typically handle these severe trailer towing conditions during the break-in period without experiencing elevated temperatures and potential damage to the axle and fluid.

[0004] In the paper by O'Connor et al., entitled Axle Efficiency—Response to Synthetic Lubricant Components (SAE Paper No. 821181), the authors state that “[i]nvestigations with both partial-and full-synthetic base formulations have shown improvements compared to conventional petroleum base gear oils. Maximum benefits are gained with total synthetic base type formulations.”

[0005] U.S. Pat. No. 4,370,247 discloses a semi-synthetic gear and axle oil composition comprising (a) 5 to 50 mass % of a conventional gear/axle grade mineral oil; (b) 5 to 30 mass % of a polyoxyalkylene glycol; and (c) 25 to 60 mass % of at least one di-C₈ to C₁₂ alkyl ester of a dicarboxylic acid. The '247 patent fails to teach or reasonably suggest the petroleum based bimodal gear oil formulations of the present invention.

[0006] U.S. Pat. Nos. 5,843,874; 5,763,372; and 5,547,596 relate to lubricant oil for gear and limited slip differential, but fail to solve the problem of achieving low axle temperature and high fuel economy.

[0007] All patents, patent applications, and articles referenced herein are fully incorporated by reference.

SUMMARY OF THE INVENTION

[0008] The bimodal distribution approach (BDA) of the present invention for formulating about 13 to about 24 cSt. viscosity hypoid axle gear lubricants is an alternate method to meet desired viscosity ranges or grades that provide fuel economy as measured by axle efficiency. Furthermore, the present invention provides temperature reduction in axles lubricated with the compositions of the present invention, especially under severe break-in conditions. The finished fluid formulation described herein provides proper axle conditioning of green axles under severe duty trailer-towing break-in conditions. The system is formulated to reduce the peak and continuous operating temperatures in green axles under trailer towing conditions.

[0009] The approach described herein uses a low viscosity base oil of about 2 cSt to about 8 cSt in combination with an undiluted viscosity index improver (VII) with a viscosity in the range of from about 600 cSt to about 45,000 cSt at 100° C. The preferred mix in one embodiment uses a 4 cSt base oil (preferably a Group II-IV base oil or mixture thereof) in combination with (a) a VII selected from an olefin copolymer (OCP) and/or with polymethacrylate (PMA) with a MW of about 12,000 or a polyisobutylene (PIB) with a MW of about 2,000 to 2,400; (b) an antiwear additive that is in one embodiment an amine derivative salt of dialkyldithiophosphoric acid; and (c) a friction modifier selected from the group consisting of the reaction products of a C₅ to C₆₀ carboxylic acid and at least one amine selected from the group consisting of (i) guanidine, urea and thiourea compounds, (ii) C₁ to C₂₀ hydrocarbyl or hydroxy-substituted hydrocarbyl (A) mono-amines, (B) alkylene diamines, and (C) polyalkylene polyamines; and (iii) N-alkyl glycine.

[0010] In an axle operating under a variety of speed and load conditions typifying consumer driving patterns, gear lubricants blended using the BDA of the present invention have shown lower operating temperatures and improved hypoid axle efficiencies, compared to the temperatures and efficiencies observed in hypoid axles lubricated with commercially available conventional gear lubricants. A feature of the present invention is to provide a bimodal hypoid axle gear lubricant formulation which addresses the vehicular fuel economy problems of gear lubricants of the prior art. More specifically, the present invention provides a bimodal gear lubricant formulation which provides improved and unexpected performance, particularly improved vehicular fuel economy performance, in gear lubricant applications relative to the vehicular fuel economy performance of conventional, non-bimodal gear lubricants.

[0011] The BDA formulations of the present invention also allow formulators the ability to increase viscosity of their finished lubricant and yet have comparable or better fuel economy than low or lower viscosity competitive gear lubricants. One significant benefit of gear lubricants based on BDA of the present invention is meeting the otherwise impossible task of improving fuel economy and reducing axle temperatures under severe towing conditions. Conventional low viscosity gear lubricants provide improved fuel economy but do not provide adequate relief under high loads observed during towing. BDA gear lubricants of the present invention blended to a higher viscosity by comparison provide both better fuel economy and lower axle temperatures.

[0012] By “bimodal” herein is meant gear lubricant formulations which generate a gel permeation chromatogram (“GPC”) curve having at least two distinct peaks, one of which GPC peaks is representative of a base oil in the gear lubricant formulation, and the other of which GPC peaks is representative of a VII. Bimodal distributions have been demonstrated to be important to improving fuel economy by improving the churning losses in the low EHD region.

[0013] The bimodal gear lubricant formulations of the present invention have a finished oil viscosity of from about thirteen centistokes to about twenty-four centistokes, more preferably of from about fifteen centistokes to about nineteen centistokes, and most preferably about seventeen centistokes when measured at 100° C.

[0014] Therefore, a preferred base oil in an embodiment of the present invention can include a natural or synthetic oil or mixture thereof having a viscosity range of from about two centistokes to about eight centistokes, and preferably about four centistokes, plus or minus about a ten percent variation.

[0015] Another feature is to provide enhanced durability of hardware and lubricant as a direct consequence of lower operating temperatures.

[0016] A further feature of the present invention is to provide a method of improving the fuel economy of a vehicle whose hypoid gears and/or transmission are/is lubricated with a bimodal gear lubricant formulation of the present invention, or a lubricating oil containing the bimodal gear lubricant formulation of the present invention.

[0017] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are intended to provide further explanation of the present invention, as claimed.

DETAILED DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a graph showing wind tunnel performance comparisons of the present invention versus a 75W-90 hypoid axle oil currently used in factory fill applications.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

[0019] A preferred embodiment of the present invention provides a bimodal hypoid gear lubricant formulation including a sufficient amount of a base oil of a lubricating viscosity, and a VII, friction modifier and antiwear additive. The desire is to achieve an acceptable finished lubricating oil of proper viscosity by combining a low viscosity base oil with a high viscosity polymeric thickener. In addition, a balanced friction modifier and antiwear system provide proper axle conditioning of green axles under severe duty trailer-towing break-in conditions.

[0020] In a more preferred embodiment, the present invention includes a base oil having a viscosity of from about two to about eight centistokes, more preferred from about three to about five centistokes, and most preferred about four centistokes.

[0021] A preferred VII in an embodiment of the present invention is selected from the group consisting of OCP, PIB, and PMA, wherein the undiluted VII has a viscosity in the range of from about 600 cSt to about 45,000 cSt. The bimodal gear formulations of the present invention have a finished oil viscosity of about 17 centistokes, but exhibit significantly improved fuel economy relative to conventional, non-bimodal gear lubricant formulations.

[0022] In a preferred embodiment of the present invention, the lubricating oil useful in the BDA formulations is selected from base oils known in the lubricant art and including mineral oils, synthetic oils, PAO oils, PMA oils, polyolefin oils such as polybutenes, polyisobutenes or PIB, and copolymers, mixtures and oligomers thereof.

[0023] If the base oil used in the present invention has a viscosity which is too low, more polymer is required, or a higher MW polymer, is required to achieve the desired finished lubricant oil viscosity of about 17 cSt. However, such alterations can create shear stability problems. If the base oil has too low viscosity, a broader distribution of GPC results and problems in the gear box are observed.

[0024] Oils of lubricating viscosity contemplated for use in the present invention include natural lubricating oils, synthetic lubricating oils and mixtures thereof. Suitable lubricating oils also include basestocks obtained by isomerization of synthetic wax and slack wax, as well as basestocks produced by hydrocracking the aromatic and polar components of the crude. In general, both the natural and synthetic lubricating oils will each have a kinematic viscosity ranging from about 1 to about 40 mm²/s (cSt) at 100° C., although typical applications will require each of the base oils to have a viscosity ranging from about 1 to about 12, preferably 2 to 8, mm²/s (cSt) at 100° C.

[0025] Natural lubricating oils include animal oils, vegetable oils (e.g., castor oil and lard oil), petroleum oils, mineral oils, and oils derived from coal or shale. A preferred natural lubricating oil herein is mineral oil.

[0026] The mineral oils useful in this invention include all common mineral oil base stocks. This would include oils that are naphthenic or paraffinic in chemical structure. Oils that are refined by conventional methodology using acid, alkali, and clay or other agents such as aluminum chloride, or extracted oils produced, for example, by solvent extraction with solvents such as phenol, sulfur dioxide, furfural, dichlorodiethyl ether, etc. They may be hydrotreated or hydrorefined, dewaxed by chilling or catalytic dewaxing processes, or hydrocracked. The mineral oil may be produced from natural crude sources or be composed of isomerized wax materials or residues of other refining processes. In a preferred embodiment, the oil of lubricating viscosity is a hydrotreated, hydrocracked and/or iso-dewaxed mineral oil having a Viscosity Index (VI) of greater than 80, preferably greater than 90; greater than 90 volume % saturates and less than 0.03 wt. % sulfur.

[0027] Group II and Group III basestocks are particularly suitable for use in the present invention, and are typically prepared from conventional feedstocks using a severe hydrogenation step to reduce the aromatic, sulfur and nitrogen content, followed by dewaxing, hydrofinishing, extraction and/or distillation steps to produce the finished base oil. Group II and III basestocks differ from conventional solvent refined Group I basestocks in that their sulfur, nitrogen and aromatic contents are very low. As a result, these base oils are compositionally very different from conventional solvent refined basestocks. The American Petroleum Institute has categorized these different basestock types as follows: Group I, >0.03 wt. % sulfur, and/or <90 vol % saturates, viscosity index between 80 and 120; Group II, ≦0.03 wt. % sulfur, and ≧90 vol % saturates, viscosity index between 80 and 120; Group III, ≦0.03 wt. % sulfur, and ≧90 vol % saturates, viscosity index >120; Group IV, poly-alpha-olefins. Hydrotreated basestocks and catalytically dewaxed basestocks, because of their low sulfur and aromatics content, generally fall into the Group II and Group III categories.

[0028] There is no limitation as to the chemical composition of the various basestocks used in the present invention. For example, the proportions of aromatics, paraffinics, and naphthenics in the various Group I, Group II and Group III oils can vary substantially. The degree of refining and the source of the crude used to produce the oil generally determine this composition.

[0029] In a preferred embodiment, the base oil comprises a mineral oil having a VI of at least 110.

[0030] The lubricating oils may be derived from refined, re-refined oils, or mixtures thereof. Unrefined oils are obtained directly from a natural source or synthetic source (e.g., coal, shale, or tar sands bitumen) without further purification or treatment. Examples of unrefined oils include shale oil obtained directly from a retorting operation, petroleum oil obtained directly from distillation, or an ester oil obtained directly from an esterification process, each of which is then used without further treatment. Refined oils are similar to the unrefined oils except that refined oils have been treated in one or more purification steps to improve one or more properties. Suitable purification techniques include distillation, hydrotreating, dewaxing, solvent extraction, acid or base extraction, filtration, and percolation, all of which are known to those skilled in the art. Re-refined oils are obtained by treating used oils in processes similar to those used to obtain the refined oils. These re-refined oils are also known as reclaimed or reprocessed oils and are often additionally processed by techniques for removal of spent additives and oil breakdown products.

[0031] Synthetic lubricating oils include hydrocarbon oils and halo-substituted hydrocarbon oils such as oligomerized, polymerized, and interpolymerized olefins; alkylbenzenes; polyphenyls; and alkylated diphenyl ethers, alkylated diphenyl sulfides, as well as their derivatives, analogs, and homologs thereof, and the like. Preferred synthetic oils are oligomers of α-olefins, particularly oligomers of 1-decene, having a viscosity ranging from about 1 to about 12, preferably 2 to 8, mm²/S (cSt) at 100° C. These oligomers are known as poly-α-olefins or PAOs.

[0032] Synthetic lubricating oils also include alkylene oxide polymers, interpolymers, copolymers, and derivatives thereof where the terminal hydroxyl groups have been modified by esterification, etherification, etc. This class of synthetic oils is exemplified by polyoxyalkylene polymers prepared by polymerization of ethylene oxide or propylene oxide; the alkyl and aryl ethers of these polyoxyalkylene polymers (e.g., methyl-polyisopropylene glycol ether having an average molecular weight of 1000, diphenyl ether of polypropylene glycol having a molecular s weight of 100-1500); and mono- and poly-carboxylic esters thereof (e.g., the acetic acid esters, mixed C₃-C₈ fatty acid esters, and C₁₂ oxo acid diester of tetraethylene glycol).

[0033] Another suitable class of synthetic lubricating oils comprises the esters of dicarboxylic acids (e.g., phthalic acid, succinic acid, alkyl succinic acids and alkenyl succinic acids, maleic acid, azelaic acid, subric acid, sebasic acid, fumaric acid, adipic acid, linoleic acid dimer, malonic acid, alkylmalonic acids, alkenyl malonic acids, etc.) with a variety of alcohols (e.g., butyl alcohol, hexyl alcohol, dodecyl alcohol, 2-ethylhexyl alcohol, ethylene glycol, diethylene glycol monoethers, propylene glycol, etc.). Specific examples of these esters include dibutyl adipate, di(2-ethylhexyl) sebacate, di-n-hexyl fumarate, dioctyl sebacate, diisooctyl azelate, diisodecyl azelate, dioctyl isothalate, didecyl phthalate, dieicosyl sebacate, the 2-ethylhexyl diester of linoleic acid dimer, and the complex ester formed by reacting one mole of sebasic acid with two moles of tetraethylene glycol and two moles of 2-ethyl-hexanoic acid, and the like. A preferred type of oil from this class of synthetic oils are adipates of C₄ to C₁₂ alcohols.

[0034] Esters useful as synthetic lubricating oils also include those made from C₅ to C₁₂ monocarboxylic acids and polyols and polyol ethers such as neopentyl glycol, trimethylolpropane pentaeythritol, dipentaerythritol, tripentaerythritol, and the like.

[0035] Silicon-based oils (such as the polyalkyl-, polyaryl-, polyalkoxy-, or polyaryloxy-siloxane oils and silicate oils) comprise another useful class of synthetic lubricating oils. These oils include tetra-ethyl silicate, tetra-isopropyl silicate, tetra-(2-ethylhexyl) silicate, tetra-(4-methyl-2-ethylhexyl) silicate, tetra-(p-tert-butylphenyl) silicate, hexa-(4-methyl-2-pentoxy)-disiloxane, poly(methyl)-siloxanes and poly (methylphenyl) siloxanes, and the like. Other synthetic lubricating oils include liquid esters of phosphorus containing acids (e.g., tricresyl phosphate, trioctylphosphate, and diethyl ester of decylphosphonic acid), polymeric tetra-hydrofurans, poly-alpha-olefins, and the like.

[0036] The base oil should have a limited molecular weight range effective to provide the desired viscosity and shear stability for a gear lubricant. If the viscosity of the base oil used is too low, an excessive and undesirable high amount of polymer in the VII is required to sufficiently increase the finished oil viscosity, or a VII polymer with a higher molecular weight is required. These potential solutions to low viscosity of base oil, however, can lead to shear stability problems. If the viscosity of the base oils is too high, an undesirably wide molecular weight distribution of the polymer can result which creates problems of lubricant fluid movement within the gear box.

[0037] The VIIs useful in the bimodal gear lubricant formulations of the present invention can include OCP, PIB, and PMA. Preferred examples of VIIs useful herein include PIB with a MW of about 2,400, and OCP with a MW of about 12,000.

[0038] The preferred VIIs useful in the present invention include, but are not limited to, olefin copolymers, polymethacrylates, and polyisobutylenes.

[0039] The antiwear additive may, in one embodiment, be selected from the class of amine derivative salts of dialkyldithiophosphoric acids. The amine portion of the salt may be derived from primary alkyl amines, tertiary alkyl amines, heterocyclic amines, anilines, alkoxy amines, amides, and derivatives of amine compounds produced by reaction of the amine compound with an additional oil soluble acidic organic compound. Examples of these derivatives of amine compounds include oil-soluble ashless dispersants having a basic nitrogen and/or at least one hydroxyl group in the molecule. Suitable dispersants include alkenyl succinimides, alkenyl succinic ester-amides, Mannich bases, hydrocarbyl polyamines, or polymeric polyamines. An important feature of the amine compound is that it have a basicity sufficient to produce a salt when contacted with the dialkyldithiophosphoric acid.

[0040] The dialkyldithiophosphoric acid portion of the salt may be selected from the class of compounds produced by reacting primary or secondary alcohol, or phenols with diphosphorous pentasulfides. These dialkyldithiophosphoric acids can be derived from primary alcohols, secondary alcohols, phenolic alcohols, or mixtures thereof.

[0041] The preferred antiwear compounds can include dialkyl dithiophosphates, alkyl phosphites, alkyl thiosphosphites; and alkyl phosphates. A more preferred embodiment is a dialkyl dithiophosphate derived from primary alcohols, secondary alcohols, or a mixture of primary and secondary alcohols. The most preferred embodiment is the reaction product of phosphorous pentasulfide with a mixture of isopropanol, isobutanol, and 2-ethylhexanol. In a preferred embodiment, the reaction product is about 1 mole of phosphorous pentasulfide mixed with about 4 moles of the alcohol mixture. More particularly, within the mixed alcohols, the preferred molar ratio is about 0.4 moles of isopropanol: about 0.4 moles isobutanol: about 0.2 moles 2-ethylhexanol.

[0042] The friction modifier useful in this invention may include the reaction products of a C₅ to C₆₀ carboxylic acid and at least one amine selected from the group consisting of (i) guanidine, urea and thiourea compounds, (ii) C₁ to C₂₀ hydrocarbyl or hydroxy-substituted hydrocarbyl (a) mono-amines, (b) alkylene diamines, and (c) polyalkylene polyamines; and (iii) N-alkyl glycine. Other friction modifiers useful in the present invention include alkyl amines, alkyl amides, alkyl imides; polyol esters, and imidazolines.

[0043] The salts of antiwear and friction modifiers can include transition metal salts of the dialkyl dithiophosphates, amine derivative salts of the dialkyl dithiophosphates; carboxylic acid salts of alkyl amines, carboxylic acid salts of polyamines; and carboxylic acid salts of polyamines and amides.

[0044] The optional dispersants useful in the present invention include oil-soluble ashless dispersant having a basic nitrogen and/or at least one hydroxyl group in the molecule. Suitable dispersants include-alkenyl-succinimides,alkenyl succinic acid-estersdalkdenyl succinic-ester-amides, Mannich bases, hydrocarbyl polyamines, or polymeric polyamines.

[0045] The bimodal gear lubricant formulations of the present invention can further include a gear additive package which typically contains one or more additives selected from the group consisting of dispersants, corrosion inhibitors, extreme pressure additives, rust inhibitors, antioxidants, deodorizers, defoamers, demulsifiers, dyes, fluorescent coloring agents and pour point depressants. The gear additive package may be, although it does not have to be, a fully-formulated gear additive package, such as a package meeting the requirements for API GL-5 and/or API MT-1 and/or MIL-PRF-2105E and/or AGMA 9005-D94. The components present in the gear additive package will depend on the intended final use of the product.

[0046] The bimodal gear lubricant formulations of the present invention are particularly suitable for use in automotive hypoid axle gear applications such as final drives, power-dividers or axles in light and heavy-duty vehicles.

[0047] Preferred finished lubricant formulations for automotive gear applications utilize components proportioned such that the lubricant formulations preferably have an SAE Viscosity Grade of at least SAE 70W, and preferably at least 75W, according to SAE J306 JUL98. The lubricant formulations may also have multi-grade ratings including SAE 75W-80, 75W-90, 80W-140. It is critical that the components used for formulating the lubricant formulations of the present invention are selected such that the formulated oil will not shear out of grade according to SAE J306 requirements when subjected to the 20-hour taper bearing shear test (CEC-L45-T-93). Preferably, the lubricant compositions of the present invention have a viscosity loss at 100° C. of less than about 15% in the 20-hour taper bearing shear test.

[0048] Preferred finished lubricant formulations for industrial gear applications utilize components proportioned such that the lubricant formulations have a viscosity classification of ISO 32 or higher according to AGMA 9005-D94.

[0049] The effectiveness of this bimodal gear lubricant can be evaluated under laboratory dynamometer rig testing conditions. An axle rig test is run under various pinion speed and load conditions and the torque transfer to the load cells located at each wheel end is measured. The axle's percentage efficiency (i.e. the percentage of power transferred to the wheels through the axle) is calculated as follows:

[0050] The test is allowed to stabilize at the specified conditions until the temperature change is less than 0.6degrees Fahrenheit per minute. The efficiency of the axle was calculated using a 30 second (prior 30s data) running average of the pinion torque.

Efficiency#1 (%)=(LRPM×LLoad+RRPM×RLoad)×2.62×100/(Pinion Torque×Pinion RPM)

[0051] 2.62 is the dynamometer load arm constant

[0052] The axle efficiency testing can be run at a variety of speeds and loads to simulate different driving conditions. Typically, a new axle is run through a 50-60 hour break-in sequence. The break-in fluid is then drained and replaced with the test fluid, which is run through a set of speed and load conditions. The speeds and loads at both wheels are measured on the dynamometers as well as the input speed and load on the pinion. One representative set of conditions is composed of fourteen stages:

[0053] Stage 1: 40 ft-lbs pinion torque; 1150 rpms pinion speed

[0054] Stage 2: 55 ft-lbs pinion torque; 2400 rpms pinion speed

[0055] Stage 3: 50 ft-lbs pinion torque; 1000 rpms pinion speed

[0056] Stage 4: 50 ft-lbs pinion torque; 2000 rpms pinion speed

[0057] Stage 5: 50 ft-lbs pinion torque; 3000 rpms pinion speed

[0058] Stage 6: 100 ft-lbs pinion torque; 500 rpms pinion speed

[0059] Stage 7: 200 ft-lbs pinion torque; 500 rpms pinion speed

[0060] Stage 8: 200 ft-lbs pinion torque; 1000 rpms pinion speed

[0061] Stage 9: 200 ft-lbs pinion torque; 2000 rpms pinion speed

[0062] Stage 10: 200 ft-lbs pinion torque; 3000 rpms pinion speed

[0063] Stage 11: 300 ft-lbs pinion torque; 500 rpms pinion speed

[0064] Stage 12: 300 ft-lbs pinion torque; 1500 rpms pinion speed

[0065] Stage 13: 400 ft-lbs pinion torque; 500 rpms pinion speed

[0066] Stage 14: 400 ft-lbs pinion torque; 1000 rpms pinion speed

[0067] An important feature of this invention is the ability of this fluid to control the temperature increase in automotive, light truck, and SUV axles under trailer towing break-in conditions. Handling high load and heat stresses during the first several thousand miles of axle operation, commonly referred to, as the axle break-in period is crucial to the long-term operation of the axle and fluid. Severe duty break-in situations are defined as vehicle operations and towing of loads without prior conditioning of the new (“green”) axle. The operating circumstances can include pinion gear speeds of between 1000-3000 rpms in combination with pinion torque loads of 180-400 ft-lbs, for extended periods of at least 60 minutes of continuous operation. Unless the green axle has been properly conditioned at lower speeds and loads for a sufficient time to break-in the axle, such extreme trailer-towing conditions can result in overheating of the axle and fluid and damage to both the equipment and the integrity of the fluid properties.

[0068] The effectiveness of a fluid's ability to reduce the axle temperature under break-in conditions has been tested in a dynamometer axle rig test designed to simulate the trailer towing break-in conditions in the field. This test has been referred to as the Wind Tunnel Test.

[0069] Wind Tunnel Test:

[0070] Testing Apparatus:

[0071] American Axle & Manufacturing non-lubricated iron beam axle with a 8.6 inch ring gear. Dynamometers capable of handling the required loads [Mid-west Dynamometers model 3232]. Engine and transmission capable of producing the proper speed and torque required for the test. An enclosed box structure surrounding the axle to simulate the airflow conditions encountered during trailer towing at various speeds. Box dimensions are: 30″ high×30″ wide×72″ deep with an opening in the rear of the box where the fan is placed.

[0072] A fan to pull air across the axle during the test capable of delivering 1500 ft/min of airflow. A data acquisition system to record the following information:

[0073] Sump temperature [Temprel J-type thermocouple]

[0074] Pinion torque [Himmelstein model 18000 in-lb 2661T (18-3)]

[0075] Left/Right axle torque [BLH Load Cell model U3G1]

[0076] Pinion speed [Himmelstein model 18000 in-lb 2661T (18-3)]

[0077] Left/Right axle speed.

[0078] Procedure:

[0079] The test axle is filled with 1930 grams of the test lubricant and placed on the dynamometer rig, and enclosed in the wind tunnel box. The fan is started to produce airflow in the wind tunnel of 1500 ft/min. The engine is started and the speed is set to produce an axle pinion speed of 2835 rpms at a pinion torque of 204 ft-lbs. These conditions are maintained for 90 minutes and data is acquired at 1.66 Hz for the sump temperature, pinion speed, pinion torque, left axle speed, left axle load, right axle speed, and right axle load. The maximum temperature recorded is noted along with the end of test temperature and the shape of the temperature profile. In addition, the efficiency of the axle under these conditions may be calculated using the following method:

[0080] The efficiency of the axle is calculated using a 30 second (prior 30s data) running average of the pinion torque.

Efficiency#1 (%)=(LRPM×LLoad+RRPM×RLoad)×2.62×100/(Pinion Torque×Pinion RPM)

[0081] 2.62 is the dynamometer load arm constant.

EXAMPLES

[0082] The following examples further illustrate aspects of the present invention but do not limit the present invention. Component Wgt % Inventive Sample A: Fully formulated gear additive package 8.75 meeting MIL-PRF-2105-E requirements Succinimide dispersant 3.00 Phosphorylated, boronated succinimide dispersant 2.00 Polyacrylate antifoamant 0.10 Amide Friction modifier 0.75 Ester seal swell agent 10.0 olefin copolymer VII 14.7 4 cSt PAO 57.7 Akyl amine salt of dialkyl dithiophosphate 1.50 Succinimide dispersant salt of dialkyl dithiophosphate 1.50 Inventive Sample B: Fully formulated gear additive package 8.75 meeting MIL-PRF-2105-E requirements Succinimide dispersant 5.00 Polyacrylate antifoamant 0.10 Amide Friction modifier 0.75 Ester seal swell agent 10.0 olefin copolymer VII 14.7 4 cSt PAO 57.7 Akyl amine salt of dialkyl dithiophosphate 1.50 Succinimide dispersant salt of dialkyl dithiophosphate 1.50

[0083] Table 1 shows the axle efficiency comparisons of Inventive Sample B versus a 75W-90 hypoid axle oil currently used in factory fill applications. The stages shown in this table correlate to the representative set of conditions discussed herein. As shown in Table 1, the Inventive Sample B provided improved axle efficiency at every measured stage, relative to the axle efficiency of the conventional factory fill 75W-90 axle lubricant. Overall, about a 1% improvement in axle efficiency has been observed. This typically translates to 0.1-0.2 miles per gallon fuel economy improvement in the Coporate Average Fuel Economy (CAFE) rating. TABLE 1 % Efficiency % Efficiency Stage 75W-90 Sample B 1 87.9 92.4 2 90.4 92.1 3 88.8 93.0 4 91.4 92.7 5 90.6 91.6 6 94.2 93.3 7 94.5 95.0 8 95.4 95.8 9 96.0 96.5 10 95.9 96.6 11 95.3 96.3 12 96.2 96.8 13 95.1 95.8 14 95.9 96.6

[0084] Table 2 and FIG. 1 both show wind tunnel performance comparisons of Inventive Samples A & B versus a 75W-90 hypoid axle oil currently used in factory fill applications. The graph and shape demonstrate the significantly improved temperature reduction properties of the invention under green axle break-in conditions. Such temperature reduction, combined with improved fuel economy and axle efficiency are novel and unexpected, but highly desired properties in an axle lubricant. The composition of the present invention lowers the maximum axle operating temperature during break-in by at least 20° F., and lowers the stabilize axle operating temperature during severe break-in conditions by at least 70° F. TABLE 2 75W-90 Inventive Sample A Inventive Sample B Maximum 389.7 355.4 343.5 Temperature (° F.) End of Test 382.0 252.5 256.2 Temperature (° F.)

[0085] Other embodiments of the present invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims. The patentee does not intend to dedicate any disclosed embodiments to the public, and to the extent any disclosed modifications or alterations may not literally fall within the scope of the claims, they are considered to be part of the invention under the doctrine of equivalents. 

What is claimed is:
 1. A hypoid axle bimodal gear lubricant formulation comprising: (a) a base oil of lubricating viscosity, (b) at least one viscosity index improver, (c) a friction modifier, and (d) an antiwear additive; wherein said bimodal gear lubricant formulation produces a gel permeation chromatogram having at least a first peak representative of the base oil and at least a second peak representative of the viscosity index improver, and wherein said base oil has a viscosity in the range of from about 2 centistokes to about 8 centistokes at 100° C., and wherein said undiluted viscosity index improver has a viscosity in the range of from about 600 centistokes to about 45,00 centistokes at 100° C.
 2. The formulation of claim 1 wherein said friction modifier is selected from the group consisting of: the reaction products of a C₅ to C₆₀ carboxylic acid and at least one amine selected from the group consisting of (i) guanidine, urea and thiourea compounds, (ii) C₁ to C₂₀ hydrocarbyl or hydroxy-substituted hydrocarbyl (a) mono-amines, (b) alkylene diamines, and (c) polyalkylene polyamines; and (iii) N-alkyl glycine.
 3. The formulation of claim 1 wherein said antiwear additive is an amine derivative salt of dialkyldithiophosphoric acid.
 4. The formulation of claim 3 wherein said amine is selected from the group consisting of primary alkyl amines, tertiary alkyl amines, heterocyclic amines, anilines, alkoxy amines, amides, and derivatives of amine compounds produced by reaction of the amine compound with an additional oil soluble acidic organic compound.
 5. The formulation of claim 4 wherein said derivative of an amine compound is selected from the group consisting of an oil-soluble ashless dispersant having a basic nitrogen, an oil-soluble ashless dispersant having at least one hydroxyl group, and an oil soluble ashless dispersant having a basic nitrogen and at least one hydroxyl group.
 6. The formulation of claim 5 wherein said dispersant is selected from the group consisting of alkenyl succinimides, alkenyl succinic ester-amides, Mannich bases, hydrocarbyl polyamines, and polymeric polyamines.
 7. The formulation of claim 3 wherein said dialkyldithiophosphoric acid is the reaction product of diphosphorous pentasulfides and at least one alcohol selected from the group consisting of a primary alcohol, a secondary alcohol, a phenol and mixtures thereof
 8. The formulation of claim 1, wherein the viscosity of the formulation is from about 13 centistokes to about 24 centistokes at 100° C.
 9. The formulation of claim 8, wherein the viscosity of the formulation is from about 15 centistokes to about 19 centistokes at 100° C.
 10. The formulation of claim 9, wherein the viscosity of the formulation is about 17 centistokes at 100° C.
 11. A lubricating oil comprising a formulation of claim
 1. 12. A gear lubricated with a formulation of claim
 1. 13. A gear lubricated with the oil of claim
 11. 14. A vehicular transmission lubricated with a formulation of claim
 1. 15. A vehicular transmission lubricated with the oil of claim
 11. 16. A vehicle comprising the gear of claim
 12. 17. A vehicle comprising the gear of claim
 13. 18. A vehicle comprising the transmission of claim
 14. 19. A vehicle comprising the transmission of claim
 15. 20. A method of lubricating a gear box, differential, or transmission comprising adding to a gear box, differential, or transmission the lubricant formulation of claim
 1. 21. A method for reducing operating temperatures during green axle break-in comprising adding to said axle the lubricant formulation of claim 1, whereby the operating temperature in said axle is reduced relative to the operating temperature in a comparable axle not lubricated with the lubricant formulation of claim
 1. 22. A method to improve fuel economy of a vehicle with an axle and gear assembly, said method comprising lubricating the axle or gear with the lubricant formulation of claim
 1. 23. A method to improve axle efficiency of a vehicle with an axle and gear assembly, said method comprising lubricating the axle or gear with the lubricant formulation of claim
 1. 